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Here is very detail information on this code: http://repairpal.com/OBD-II-Code-P0172-and-P0175
If you've never done this before, then you need to invest in a repair manual with step-by-step instructions and diagrams. You can get access to one online for cheap: http://bit.ly/mitchell_repair_manuals_online This can be tricky and sometimes requires special tools, so you will likely be better off taking it to a shop. It won't be THAT expensive.
Chevron has a fuel system additive called Techron Concentrate Plus. The formula with the "Plus" is specifically designed to help with this very common GM problem--it's caused by corrosion on the fuel sending unit in the gas tank. If a couple of tanks with the Techron Concentrate doesn't work, then you are probably out a bundle for the fuel sending unit.
The dealership will have Recalls relevant to your vehicle searchable your VIN. Not all Focus are necessarily affected by each Recall campaign number. Your recourse is to ask the deal what Recalls have been issued for your VIN. If you are uncomfortable with they way they are treating you, then I would call Ford itself and ask them about your vehicle. Your state may also have an auto repair consumer protection arm, as in California with the Bureau of Auto Repair.
This is strange behavior and a serious safety issue. I strongly recommend you take the car to a professional for thorough diagnosis and repair.
I strongly recommend taking your car to a professional technician for help with this problem. Fuel pump is a total guess at this point--you are going to put serious time and money into trying to fix this, and the money at a shop would be well spent.
It's probably not a vacuum problem. Is the motor running properly? You likely have a transmission problem.
This content may be some help to you. It could be a number of things: http://repairpal.com/parking-brake-warning-light
It's likely that you'll need a new clutch slave and master cylinder. You can look up estimate ranges for both with RepairPal's estimator: http://repairpal.com/estimator
Please take this to a shop for professional help from a technician certified in AC repair. You'll save yourself time, frustration, and potential safety risks.